Check the part number on the back of the rear derailleur, Circa 1994-1995, RX100 was upgraded to 8 speed and got a new derailleur that was 7/8 speed compatible. Its part number was RD-A551. The previous version, RD-A550 was designed as a 6/7 speed derailleur. It may or may not have an issue.
Unfortunately, it's not simply an issue of going from 126mm to 130mm. Things like stay and dropout design come into effect, as they affect the distance of the derailleur from the cassette or freewheel. This is why some members will report a successful conversion with a particular derailleur while another reports a failed attempt. The frame design is an often overlooked factor that comes into play.